not subject to decay or death; imperishable, beyond the reach of ruin
At its heart this word is a negation: it names whatever the ordinary process of wearing out, rotting, and dying cannot touch. The ancient world took for granted that everything in the visible order eventually breaks down; to call something imperishable was to set it outside that whole process. Its plain opposite is φθαρτός, "perishable," and Paul lays the two side by side when he contrasts the imperishable God with the perishable images people worshiped instead (Romans 1:23).
The word points in two directions that share one core. Of God it means deathless and undecaying by his very nature: he is the one being who was never going to break down or die. Of everything else it names a quality God gives, the imperishable life, inheritance, and reward that belong to the age to come, held out to people who by nature do perish.
The related noun ἀφθαρσία names the state itself, imperishability, while this adjective describes what has it. Scripture also stacks it with neighboring promises, calling the believer's inheritance imperishable, undefiled, and ἀμάραντος, "unfading," so that several angles on one hope reinforce each other.